Device and Method for Evacuating a Chamber and Purifying the Gas Extracted From Said Chamber

ABSTRACT

A device evacuates a chamber and purifies the gas extracted from said chamber of any foreign substances. The device comprises a dry-condensing vacuum pump having an input connected to the chamber to be evacuated and is suitable for maintaining the input pressure at a constant level at the output despite variable conditions. An intermediate line which connects to the output of the dry-condensing vacuum pump and a liquid ring vacuum pump, the input of which is connected to the intermediate line, are additionally provided. A corresponding method makes it possible to purify the gas of any foreign substances reliably and effectively.

BACKGROUND

The invention relates to a device by means of which a chamber can beevacuated and the gas that is extracted from the chamber can be purifiedof entrained foreign substances. The device comprises a dry-compressionvacuum pump, the inlet of which is connected to the chamber to beevacuated. The vacuum pump is suitable for keeping the inlet pressureconstant despite variable conditions at the outlet, in particulardespite variable pressure at the outlet. The invention also relates to acorresponding method.

Such devices may be used for example in deposition processes, such as,for example, CVD (Chemical Vapor Deposition) processes. CVD processesare utilized in many industrial sectors to produce coatings on differentsubstrates at reduced pressure. For this purpose, gaseous metalcompounds are converted into the desired coating system on the substratesurface at high temperatures and/or with plasma assistance. Therelatively high gas pressures that can be realized permit high growthrates and have thus become widely used in all sectors of vacuum coatingtechnology. A disadvantage of said high growth rates is that thedeposition reactions take place not only on the substrate, but also inall regions of the coating installation. This applies not only to theprocess chamber walls, but also to the lines and devices between theprocess chamber and the outlet for the waste gases. Furthermore, thegaseous outlet and intermediate products are generally toxic andcorrosive, and such CVD installations are thus always also equipped witha corresponding waste-gas treatment arrangement.

In modern CVD installations, to remove the deposits in the processchamber, etching processes are performed between the process steps,which etching processes convert the deposits in the process chamber intoeasily evaporable substances. A typical representative of saidsubstances is ammonium chloride which tends to repeatedly build up onthe walls further along the line. This applies in particular to thevacuum pump and to the pressure side of the vacuum pump, because here,in accordance with the law of continuity, the volume flow rate decreasessignificantly and thus makes it possible for the solid substances tobuild up.

To solve the problems resulting from this, there are nowadays basicallytwo solution strategies used in industrial installations. One strategyis the installation of cooling traps downstream of the process chamber,with a heated line to the cooling trap, in order to accumulate thecondensable substances at one location. This approach has thedisadvantage that the cooling traps must be cleaned/evacuated at regularintervals. The other strategy is intensive heating of the lines and theadding of extensive amounts of inert gas in order to achieve an adequatevolume flow rate even on the pressure side of the vacuum pump.Disadvantages here are the energy consumption and the thermal andthermally corrosive loading of the pipelines, valves, etc. Furthermore,the inert gas flow places a burden on the waste-gas purificationarrangement, which is generally composed of a waste-gas scrubber.Furthermore, waste-gas scrubbers are highly susceptible to blockage atthe gas inlet, because the temperature drops significantly here in thecase of a high pressure and low volume flow rate.

Taking the prior art mentioned in the introduction as a starting point,a device and a method is proposed by which the gas that is extractedfrom the chamber can be purified in an expedient and reliable manner.

In many applications, it is highly important that the pressure in thechamber to be evacuated (for example the process chamber of the CVDprocess) can be kept reliably constant. The chamber thus has connectedto it a dry-compression vacuum pump, which can perform this task even ifthe conditions at the outlet of the pump cannot be reliably keptconstant. Basically, a pump is suitable for keeping the inlet pressureconstant despite variable conditions at the outlet only if the pumpcomprises a multiplicity of working chambers arranged in series. Theinlet and outlet are then separated from one another by a multiplicityof sealing gaps. An example of a pump which can perform this task is ascrew-type vacuum pump. An example of a pump which is not suitable forkeeping the inlet pressure constant despite variable conditions at theoutlet is a Roots-type pump. Here, the inlet and outlet are separatedfrom one another by only one sealing gap, and therefore a change at theoutlet has a direct effect on the conditions at the inlet owing to theleakage flow. Roots-type pumps are thus typically used as forepumps inan arrangement comprising multiple pumps. Conventional pump arrangementshaving a Roots-type pump as a forepump, such as are described, forexample, in FR 1129872 and in U.S. Pat. No. 3,956,072, have nothing todo with the invention.

A liquid ring vacuum pump is connected to the outlet of thedry-compression vacuum pump via an intermediate line. The liquid ringvacuum pump would not be suitable for being directly connected to thechamber to be evacuated, for two reasons. Firstly, the liquid ringvacuum pump cannot generate a vacuum lower than the vapor pressure ofthe operating liquid. A lower pressure is, however, often required inthe chamber to be evacuated. Secondly, a vapor partial pressure canescape from the operating liquid of the liquid ring vacuum pump in thedirection of the suction side. This is not acceptable, because it mustbe possible to maintain defined conditions in the chamber to beevacuated.

SUMMARY

In the chamber to be evacuated, an atmosphere prevails in which thesolid substances are dissolved in the gas. To prevent the solidsubstances from condensing and being precipitated on elements of thepump as the gas passes through the device, it is firstly necessary forthe pressure of the gas to be kept low, and it is secondly necessary forthe volume flow rate to be kept high, such that the gas moves at anadequate speed.

Against this background, the dry-compression vacuum pump is used toincrease the pressure, but to a value that is still considerably lowerthan atmospheric pressure. Thus, in the intermediate line, the pressureis low enough, and the speed high enough, that condensation does not yettake place. Condensation occurs for the first time in the liquid ringvacuum pump. Here, the substances can however be immediately absorbed bythe operating liquid of the liquid ring vacuum pump and transportedaway, such that as a result, undesired deposits do not form. Theexpression “condensation” encompasses in particular a situation in whicha substance changes directly from the gaseous phase into the solidphase. A further waste-gas scrubber downstream of the liquid ring vacuumpump is not required in the device.

The pressure at the inlet side of the dry-compression vacuum pump mayfor example lie between 1 mbar and 40 mbar, preferably between 2 mbarand 30 mbar, and more preferably between 5 mbar and 20 mbar. Thepressure in the intermediate line may for example lie between 80 mbarand 300 mbar, preferably between 100 mbar and 150 mbar.

A further possibility for counteracting the condensation resides in ascavenging gas being fed in at the outlet of the dry-compression vacuumpump and/or in the intermediate line. Feeding the scavenging gas in saidregion has the advantage that a considerable increase in the volume flowrate, and thus speed, can be achieved with a small amount of gas. Theopening for the feed of the scavenging gas may be a suitably arrangedvalve. It is also possible for the scavenging gas to be fed through anopening which is formed in the dry-compression vacuum pump between ashaft and the housing and which must be sealed off in any case. In thiscase, the scavenging gas can simultaneously serve as barrier gas of thedry-compression vacuum pump.

The liquid ring vacuum pump is preferably provided with an inlet and anoutlet for the operating liquid. Said inlet and outlet are furthermorepreferably designed so as to permit an inflow and an outflow of theoperating liquid during the operation of the pump. This is desirable inorder that operating liquid that has been enriched with foreignsubstances can be replaced with fresh operating liquid. In thisembodiment, the pump may be designed such that the housing of the liquidring vacuum pump is provided with an inlet opening for the inflow of theoperating liquid and with an outlet opening for the outflow of theoperating liquid. Configurations are also possible in which theoperating liquid initially emerges from the pump through the same outletas the gas flow. The operating liquid can be separated from the gas flowin the subsequent line.

There are numerous possibilities for the flow of the operating liquid.The liquid that is discharged from the liquid ring vacuum pump may bedisposed of and replaced with a corresponding amount of fresh liquid.Also conceivable is a circuit in which the liquid discharged from theliquid ring vacuum pump is partially or entirely fed back to the pump.Provision may also be made for the heat of compression to be dissipatedby means of an installed heat exchanger.

In all cases, it is desirable for the operating liquid to be disposed ofonly when it has become sufficiently enriched with foreign substances.For this purpose, provision may be made for the conductivity of theoperating liquid to be determined by means of a sensor. The content offoreign substances can be inferred from the conductivity. The sensor maybe arranged for example on the liquid ring vacuum pump or on a line oron a vessel through which the operating liquid is conducted. The devicemay be designed such that operating liquid is fed in and discharged onlyin required amounts in a manner dependent on the content of foreignsubstances.

The operating liquid may be water. It is also possible for the operatingliquid to contain a solvent which is coordinated with the foreignsubstances contained in the gas. Such a solvent can contribute toparticularly effective purification of the gas.

In the gas, there may sometimes be entrained substances which are eitheracidic or alkaline. In this case, for the purification of the gas, itmay be expedient for the operating liquid to correspondingly beopposingly alkaline or acidic. For this purpose, a pH sensor may beprovided in order to determine the pH value of the operating liquid.Furthermore, a control device may be provided which adjusts the pH valueas a function of the measurement values from the pH sensor. This may berealized, for example, by virtue of water being added to the operatingliquid if it is sought merely to neutralize the operating liquid. It isalso possible for an acid or an alkaline solution to be fed to theoperating liquid if the pH value is to be adjusted in the correspondingdirection.

Some foreign substances in the gas can best be eliminated by combustion.For this purpose, a combustion device may be provided which ispreferably arranged in the intermediate line. Said combustion device mayoperate with thermal combustion, that is to say with a flame fed withnatural gas, for example, which flame is brought into contact with thegas. Also possible is a catalytic combustion in which the gas is broughtinto contact with a heated surface composed of a catalytic material. Bymeans of the combustion, the foreign substances in the gas can bebrought into a form in which they are absorbed by the operating liquid.

The invention also relates to a method for evacuating a chamber and forpurifying the gas that is extracted from the chamber of entrainedforeign substances. In the method, gas is drawn out of the chamber bysuction. The pressure is increased to a value below atmosphericpressure, wherein a pump is used which is suitable for keeping the inletpressure constant even if the outlet pressure is variable. The gas isconducted onward to a liquid ring vacuum pump which outputs the gas atatmospheric pressure. In the method, the pressure and the volume flowrate of the gas that is conducted onward to the liquid ring vacuum pumpare set such that condensation of the foreign substances does not yettake place. The foreign substances condense for the first time in theliquid ring vacuum pump, and are absorbed by the operating liquid in thelatter and discharged. The method may be refined with further featuresthat have been described above with reference to the device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example below with referenceto the appended drawing and on the basis of advantageous embodiments. Inthe drawing:

FIG. 1 shows an embodiment of the device.

DETAILED DESCRIPTION

A process chamber of a CVD process forms the chamber 14 to be evacuated.The inlet 17 of a screw-type vacuum pump 15 is connected to the chamber14 to be evacuated. A pressure reduction valve 16 is arranged in theline between the chamber 14 to be evacuated and the screw-type vacuumpump 15, by means of which pressure reduction valve the pressure in thechamber 14 can be set to be higher than the pressure at the inlet 17 ofthe screw-type vacuum pump 15. In the present example, a pressure of 80mbar prevails in the chamber 14, and a pressure of 20 mbar prevails atthe inlet 17 of the screw-type vacuum pump 15. The screw-type vacuumpump 15 has the task of keeping the pressure at the inlet 17 constanteven if the conditions at the outlet 18 of the screw-type vacuum pump 15vary. By means of the screw-type vacuum pump 15, the gas is compressedto a pressure of approximately 120 mbar and output via the outlet 18.

The outlet 18 of the screw-type vacuum pump 15 is adjoined by anintermediate line 19 which leads to a liquid ring vacuum pump 20. Theliquid ring vacuum pump 20 compresses the gas further such that it canbe output at atmospheric pressure via the outlet 21.

The screw-type vacuum pump 15 and the liquid ring vacuum pump 20 arecoordinated with one another such that, in the intermediate line 19, thepressure of the gas is low enough, and the volume flow rate high enough,that no condensation of the foreign substances entrained by the gastakes place therein. Condensation occurs the first time as a result ofthe further pressure rise in the liquid ring vacuum pump 20. The foreignsubstances are then immediately absorbed by the operating liquid of theliquid ring vacuum pump 20 and consequently cannot be deposited onelements of the device.

Also arranged in the intermediate line 19 is a valve 22 via which airfrom the environment can be admitted, as scavenging gas, into theintermediate line 19. By virtue of scavenging gas being admitted at thislocation, a small volume of scavenging gas is sufficient to considerablyincrease the volume flow rate in the intermediate line 19. Thedeposition of the foreign substances can thus be counteracted further bymeans of the scavenging gas.

During the course of operation, the operating liquid is enriched with anever-increasing amount of foreign substances. It is thus possible fornew operating liquid to be fed to the liquid ring vacuum pump 20 and forold operating liquid to be discharged from the liquid ring vacuum pump,respectively, via an inlet 23 and an outlet 24 during operation. Here, aclosed circuit for the operating liquid exists between the liquid ringvacuum pump 20 and a storage vessel 25 via a feed line 26 and a returnline 27. Accordingly, during operation, operating liquid is continuouslyexchanged, whereby the material present in the storage vessel 25 isenriched with foreign substances to an ever greater extent.

The conductivity of the operating liquid in the storage vessel 25 iscontinuously measured by means of a sensor 28. The content of foreignsubstances can be inferred from the conductivity, such that theoperating liquid can cease to be used when a predefined threshold valueis exceeded.

The measurement values from the sensor 28 are fed to a control unit 30.If the threshold value is exceeded, the control unit 30 actuates a valve31, such that some of the no longer usable operating liquid is extractedfrom the vessel 25. The vessel 25 is subsequently filled with acorresponding amount of fresh operating liquid. Furthermore, by means ofthe control unit 30, a pump 32 is actuated by means of which theoperating liquid is fed to the liquid ring vacuum pump 20.

The pH value of the operating liquid in the storage vessel 25 ismeasured by means of a further sensor 29. If, for example, acidicforeign substances are entrained in the gas, it may be advantageous, forthe purification of the gas, for the operating liquid to be alkaline.The absorption of the acidic foreign substances has the effect that thepH value of the operating liquid falls until, at some point, theabsorption of the acidic foreign substances is no longer ensured. Thisis determined by means of the sensor 29. When a correspondingmeasurement value is output by the sensor 29, the device 30 actuates avalve 33 by means of which additional alkaline solution is fed to theoperating liquid. In this way, the operating liquid permanentlymaintains the desired alkaline characteristics.

If the foreign substances entrained in the gas are alkaline, theprocedure using the sensor 29, the control device 30 and the valve 33 isexactly reversed.

Finally, a combustion device 34 is arranged in the intermediate line 19.Said combustion device is activated if foreign substances that can beabsorbed by the operating liquid of the liquid ring vacuum pump 20 onlyafter combustion are entrained in the gas.

1. A device for evacuating a chamber and for purifying a gas that isextracted from the chamber of entrained foreign substances comprising:a. a dry-compression vacuum pump having a chamber to be evacuated, aninlet connected to the chamber to be evacuated and an outlet and whichis suitable for keeping the inlet pressure constant despite variableconditions at the outlet; b. an intermediate line which connects to theoutlet of the dry-compression vacuum pump; and c. a liquid ring vacuumpump, having an inlet connected to the intermediate line.
 2. The deviceas claimed in claim 1, characterized in that the dry-compression vacuumpump is designed to generate a pressure of between 1 mbar and 40 mbar atthe inlet side.
 3. The device as claimed in claim 1, characterized inthat the dry-compression vacuum pump is designed to generate a pressureof between 80 mbar and 300 mbar in the intermediate line.
 4. The deviceas claimed in claim 1, characterized in that an opening for the supplyof scavenging gas is provided at the outlet of the dry-compressionvacuum pump and/or in the intermediate line.
 5. The device as claimed inclaim 4, wherein the dry-compression pump has a housing characterized inthat the opening is a gap that exists between a shaft and the housing ofthe dry-compression vacuum pump.
 6. The device as claimed in claim 1,characterized in that the liquid ring vacuum pump has an operatingliquid and is provided with an inlet and an outlet for the operatingliquid, which inlet and outlet permit an inflow and outflow of theoperating liquid during the operation of the liquid ring vacuum pump. 7.The device as claimed in claim 1 wherein the operating liquid has aconductivity, characterized in that a sensor is provided for determiningthe conductivity of the operating liquid.
 8. The device as claimed inclaim 6 wherein the operating liquid has an inflow and an outflow and aconcentration of foreign substances, characterized in that a controldevice is provided which adjusts the inflow and the outflow of theoperating liquid as a function of the concentration of foreignsubstances in the operating liquid.
 9. The device as claimed in claim 1,characterized in that the operating liquid of the liquid ring vacuumpump contains a solvent which is coordinated with the foreign substancescontained in the gas.
 10. The device as claimed in claim 1 wherein theoperating liquid has a pH value, characterized in that a pH sensor isprovided for determining the pH value of the operating liquid.
 11. Thedevice as claimed in claim 10 wherein the sensor determines measurementvalues, characterized in that a control device is provided which isdesigned to adjust the pH value of the operating liquid as a function ofthe measurement values from the pH sensor.
 12. The device as claimed inclaim 1, characterized in that a combustion device is arranged in theintermediate line.
 13. A method for evacuating a chamber having a gaswith entrained foreign substances and for purifying the gas that isextracted from the chamber of entrained foreign substances, having thefollowing steps: a. drawing the gas having a pressure out of thechamber; b. increasing the pressure to a value below atmosphericpressure, wherein a pump is used which pump has an inlet pressure and anoutlet pressure and is suitable for keeping the inlet pressure constanteven if the outlet pressure is variable; c. conducting the gas onward toa liquid ring vacuum pump which outputs the gas at atmospheric pressure.14. The device as claimed in claim 1, characterized in that thedry-compression vacuum pump is designed to generate a pressure ofbetween 2 mbar and 30 mbar at the inlet side.
 15. The device as claimedin claim 1, characterized in that the dry-compression vacuum pump isdesigned to generate a pressure of between 5 mbar and 20 mbar at theinlet side.
 16. The device as claimed in claim 1, characterized in thatthe dry-compression vacuum pump is designed to generate a pressure ofbetween 100 mbar and 150 mbar in the intermediate line.
 17. The deviceas claimed in claim 2, characterized in that the dry-compression vacuumpump is designed to generate a pressure of between 100 mbar and 150 mbarin the intermediate line.
 18. The device as claimed in claim 2,characterized in that an opening for the supply of scavenging gas isprovided at the outlet of the dry-compression vacuum pump and/or in theintermediate line.
 19. The device as claimed in claim 3, characterizedin that an opening for the supply of scavenging gas is provided at theoutlet of the dry-compression vacuum pump and/or in the intermediateline.
 20. The device as claimed in claim 2, characterized in that theliquid ring vacuum pump has an operating liquid and is provided with aninlet and an outlet for the operating liquid, which inlet and outletpermit an inflow and outflow of the operating liquid during operation ofthe liquid ring vacuum pump.